1. Field of the Invention
The present invention relates to an active suspension system for an automotive vehicle. More specifically, the invention relates to a fluid circuit for supplying and draining a working medium fluid through the active suspension system.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987 which has been assigned to the common owner of the present invention, discloses one of typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder communicates with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as a proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit to be control the valve position. The pressure control valve position is controlled by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and thereby controlling suspension characteristics.
On the other hand, European Patents 0 283 004, 0 285 153 and 0 284 053 disclose technologies for controlling suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
One typical construction of the hydraulic circuit includes a pressure source unit which comprises a fluid pump drivingly associated with an automotive internal combustion engine so as to be driven by the engine output torque. The fluid pump is generally rated to produce rated pressure which is selected in view of the required line pressure in a supply line for supplying the pressurized fluid to the working chamber, at the minimum revolution speed of the engine so that the working fluid pressure to be supplied to the working chamber of the hydraulic cylinder can be satisfactorily high at any engine driving range. As will be appreciated, the output pressure of the fluid pump increases according increasing engine revolution speed. Therefore, at a high engine revolution speed range, excessive pressure in excess of a predetermined maximum line pressure is relieved via a relief valve. Therefore, the engine output can be wasted to degrade engine driving performance as a power plant for the automotive vehicle and thus degrade fuel economy.
On the other hand, in the practical operation of active suspension system, the fluid pressure in the working chamber in the hydraulic cylinder can be maintained at a constant value for maintaining a desired vehicular height, at substantially low vehicle speed range or while the vehicle is not running. Despite this fact, the prior proposed hydraulic circuits for actively controlled suspension systems supply the rated pressure of the fluid pump which should be higher than a minimum line pressure required for adjustment of the fluid pressure in the working chamber. In order to maintain the rated pressure output from the fluid pump, substantial engine output will be consumed even at a low vehicle speed range, in which line pressure is not required there being no possibility that adjustment of the suspension characteristics would be necessary.
Improvement in the hydraulic circuit for the prior proposed active suspension system has been proposed in co-pending U.S. patent application Ser. No. 331,602, filed on Mar. 31, 1989, which application has been commonly assigned to the common assignee to the present invention now U.S. Pat. No. 4,911,468. The corresponding invention to that the in above-identified co-pending U.S. Patent Application has been published as Japanese Patent First (unexamined) Publication (Tokkai) Heisei 1-249509, published on Oct. 4, 1989. The prior invention is directed to a hydraulic circuit for an actively controlled suspension system which employs first and second pressure relief valves disposed in a hydraulic pressure source circuit for relieving excessive pressure. The second pressure relief valve for a lower relief pressure than that of the first pressure relief valve. Means for selectively connecting and disconnecting the second pressure relief valve is disposed in the hydraulic pressure source circuit at a position upstream of the second pressure relief valve. This means is positioned in shut-off position to disconnect the second pressure relief valve when vehicle traveling speed is higher than a predetermined speed. This means is responsive to a vehicle speed lower than the predetermined speed for establishing a connection between a pressurized fluid source to the second relief valve for relieving the pressure at a lower level than that established when the vehicle speed is higher than the predetermined speed.
Furthermore, the prior proposed invention includes a pilot pressure operated operational one-way check valve in a drain line for regulating line pressure to be supplied to a pressure control valve which adjusts fluid pressure in a working chamber in a hydraulic cylinder disposed between a vehicle body and a suspension member rotatably supporting a road wheel, by draining excessive line pressure. Similar hydraulic circuit arrangements have also been disclosed in European Patent First Publications Nos. 0 318 721, 0 318 932, for example.
Such a prior proposed hydraulic circuit does provide improved characteristics for the active suspension system in certain aspect. However, the prior proposed system still encounters a drawback in the response characteristics of the pressure control valve unit due the presence of back pressure due to flow resistance in the drain line.
For preventing this, U.S. Pat. application Ser. No. 454,785, filed on Dec. 26, 1989, now U.S. Pat. No. 4,982,979 issued on Jan. 8, 1991, discloses a hydraulic circuit construction which can absorb back pressure in a drain line. This hydraulic circuit for an active suspension system employs a pressure accumulator connected to a drain line at a position upstream of a pilot operated operational one-way check valve. The pressure accumulator absorbs back pressure generated in the drain line due to flow resistance in the drain line. Such a hydraulic circuit construction is effective for avoiding the influence of back pressure induced in the drain line and thus enhances drain characteristics for providing better response characteristics in the active suspension system.
In such a prior proposed system, the pressure accumulator disposed in the drain line is set at a gas pressure slightly lower than the possible minimum pressure of the working fluid pressure in the hydraulic circuit. In practice, the gas pressure is set at 80 to 90% of the possible minimum hydraulic pressure so that pressure transfer medium, such as piston, diaphragm or so forth, would not be damged by collision with the inner periphery of the accumulator housing even at the minimum fluid pressure in the drain line. As can be appreciated, in view of practical installation on the vehicular body, the drain line piping is inevitably long and has a small path area to serve as resistance against the fluid flow through the drain line. Therefore, when valve position in the pressure control valve is abruptly changed to a cause substantial reduction of the fluid flowing into the drain line, the fluid pressure in the drain line can be lowered to a level lower than the gas pressure. In such case, the pressure accumulator becomes ineffective for absorbing surge pressure in the drain line. Therefore, the surge pressure may affect the control pressure supplied to the working chamber of active cylinder to cause degradation of the riding comfort. Furthermore, such surge pressure may cause collision of the pressure transferring medium with a stopper to shorten life thereof. Particularly, in case of fail-safe mode operation selected in response to failure of the fluid pressure source unit, blocking of fluid communication between the fluid pressure source unit and the pressure control valve so that the supply line is in communication with a drain line incorporating a supply line pressure responsive operational check valve is provided for maintaining the drain pressure at a level higher than a predetermined pressure, the surge pressure caused due to relatively large flow resistance in the drain piping may be fed back to the control line to unnecessarily stiffen the suspension to cause degradation of riding comfort. Furthermore, substantially high surge pressure may serve as a cause of damping of the components of the active suspension system.